Waveguide system having mode converter for changing rectangular te10 mode into circular te01 at locus of waveguide window



Aug. 13, 1963 w. A. EDSON 3,100,881 WAVEGUIDE SYSTEM HAVING MODE CONVERTER FOR CHANGING RECTANGULAR TE MODE INTO CIRCULAR TE, AT LOCUS OF WAVEGUIDE WINDOW Filed Oct. 19, 1960 2 Sheets-Sheet 1 lNV EN TOR. WILLIAM A. EDSON HIS ATTORNEY 3,100,881 R CHANGING AT Locus OF Aug. 13, 1963 w. A. EDSON WAVEGUIDE SYSTEM HAVING MODE CONVERTER FO RECTANGULAR TE MODE INTO CIRCULAR TE WAVEGUIDE WINDOW.

2 Sheets-Sheet 2 Filed Oct. 19, 1960 INVENTORI WILLIAM A. EDSON mfl rf FIG.4.

HIS ATTORNEY.

This invention relates to barriers for pressurized or evacuated waveguides. That is, the invention relates to window barrier sections which separate two regions in the waveguide or waveguide system and provide for the transmission of electromagnetic waves between the two regions.

The barriers or seals of the type under consideration .are particularly useful in the input and output waveguide connections of an electron tube which operates at microwave frequencies. In such applications electromagnetic energy must be transmitted between the evacuated inte rior of the tube envelope and exteriorwaveguide systems which may be maintained at atmospheric or other pressure. Other examples of such windows are found in applications where a barrier is required to prevent the escape of gas from a pressurized system or where a barrier is required to contain a cooling or insulating fluid in a waveguide system. These barriers which are called microwave windows have taken various forms using a variety of metal and dielectric materials.

' The use of microwave windows introduces a number of electrical and mechanical problems. The electrical problems include the introduction of reflections at the dielectric barrier due to tlie discontinuity in the medium in which the electromagnetic waves must travel, voltage breakdown in the presence of high electric and magnetic fields in the area of the dielectric barrier and the dissipation of microwave power within the dielectric material. The mechanical problems encountered include difiiculty in providing a leak proof mechanical design which can withstand the elevated temperatures required for processing and operation, has the mechanical strength to maintain line dimensions at correct values and is capable of reproduction in quantity with uniformity in dimensions and properties. i r

The electrical problems involved may best be understood by considering a particular application for microwave windows of the type under consideration. As an example consider a window used for transmission of microwave energy from a high power source, such as a klystron, to an external waveguidesystem. An output window for such generatorsmust be capable of transmitting the generated power to the waveguide system with a minimum of reflection and absorption of energy. Power absorbed at the window or reflected by the window is lost and reflected energy may severely damage the power source. Power absorption in a window of a given geometry is a function of the dielectric material and may be determined by selecting the material. Reflections in a transmission line are caused by discontinuities along the length of the line which represent abrupt discontinuities in impedance. Such discontinuities may be caused by a dielectric position in the transmission line or they may be caused by a change in the physicaldimensions of the transmission line or both. The transmission line window and support structures described herein have a physical configuration which provide a good impedance match over a broad band of frequencies and thus reduce reflections to a minimum.

Among the most serious problems encountered when using microwave windows is the actual mechanical failure Patent ice 3,100,881 Patented Aug. 13, 1963 dielectric barrier may fail as a result of over heating around the seal, there may be a thermal breakdown resulting in decomposition'of the dielectric material at high levels of average power .due to power dissipation in the window or there may be a cracking or puncture of the dielectric material from arcing or non-uniform heat ing in the region of the barrier. Such failures can gener: ally be attributed to electrical phenomena. That is, they are generally believed to be the result of voltage gradients due to the electric and magnetic field configuration in the area of the windows.

Accordingly it isthe object of the present-invention to provide a microwave transmission and window section having a structure which provides electric and magnetic field configurations in the area of the window that tend to increase the power handling capabilitiy of the section and minimize the problems of arcing, thermal breakdown, cracking, punctures, and other failures in the regions of the dielectric barrier.

Although the mechanical problems involved in micro wave window design are easier to understand than the electrical problems they are no less important. For example, if the design of the window and its associated support is not such as to permit a strong vacuum tight joint, the barrier is ineffective. Also if the. mechanical strength of the window is not sufiicient to maintain dimensions undernormal operating conditions and at the elevated temperatures required for processing, the advantages of a careful design are lost. Further, unless the designis capable of production and reproduction in quan tity with uniformity in dimensions and properties the usefulness is reduced considerably. The mechanical design of the window section of the present invention is particularly effective in that it does provide a barrier which separates two regions in the transmission line system, presents a simple strong vacuum tight mechanical structure and produces a minimum of reflection over a wide band of frequencies.

Briefly stated,in accordance with this invention a microwavef window and transition section is formed by converting incident microwaveenergy into the circular transverse electric waveguide mode which has electric field lines that are circular and concentric throughout the length of the waveguide before the microwave energy passes through the dielectric barrier of the window sec tion and providing a stmcture for the section which presents complete electrical and mechanical symmetry to the incident electromagnetic Waves. The circular waveguide mode referred to is usually designated as the TE mode. Thus, the circular TE mode is one where there isfno radial component of electric field in the waveguide and the circular component has no variation with respect to the angular coordinate.

The novel features which are believed to be character istic of the invention are set forth in the appended claims. The invention itself, however, both as to its organization and method of operation together with other objects and advantages may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a partially broken away central vertical longitudinal section taken through a portion of a window section illustrating a. particular type of mode launcher which may be used;

FIGURE 2 is a partially broken away perspective view g FIGURES 4 and 6 are central vertical longitudinal secsupport section of FIGURE 4.

Referring specifically to FIGURES land Z of the drawings a portion of a window section in constructed in accordance with the present invention is seen. For simplicity only a portion of the window section it) is illustrated and the systems. joined by the window section are omitted. As illustrated the window section includes a cylindrical pipe-like window-supporting section or structure 11'and' a mode launcher section 12 designed to launch the circular transverse electric mode designated as the TE mode in the circular waveguide section 11. As illustrated, the circular waveguide window-supporting section 11 is in effect closed at opposite ends by circular conductive' end walls 13 which are provided with apertures 14 (described in detail subsequently) for admission and extraction ofelectromagnetic energy. In order to provide a pressure barrier in the window-supporting section 11 a flat dielectric disc 15 is positioned mid-way between the end Walls 13 which close the cylindrical supporting section 11.

The dielectric disc '15 is sealed in vacuum tight relation around its outer periphery to the inner circumference of the cylindrical section 11. In the embodiment illustrated, the dielectric barrier 15 is positioned between two ,inwardly projecting conductive flanges l6 and 17 which are designed to produce an electrical reflection which adds to that produced by the dielectric slab 15 in such a way as to balance out reflections in the window section length of the cylindrical supporting section on either side of the dielectric barrier (from'tend wall 13 to dielectric barrier 15) is approximately one half wavelength for the TE mode at the frequency of interest.

It is believed that the construction of a cylindrical window supporting section 11 such as the one illustrated is well within the skill of one versed in the art. However, the interest of completeness it is noted that the section illustrated is made up of two circular waveguides 18 and 19 each having one of the conductive endplates i3 brazed at "one end and having their open ends brazed together i to form a continuous cylindrical section.

The conductive reflective window supporting flanges 16 and 17 are formed on the two circular waveguides 18and 19 respectively. That is, the conductive flange 17 on one side of the window projects inwardly from the open end tions 18 and 19 brought together and brazed to form theunitary cylindrical window-supporting section 11.

As previously indicated the end plates 13'which essentially close opposite ends of the cylindrical window supporting section 111 are each provided with a pair of rectangular apertures 14. These apertures are located to launch or excite the transverse electric TE circular waveguide mode in the cylindrical window-supporting section 11. In order to accomplish this, the apertures 14 are symmetrically located on opposite sides of the center of the ,discs or plates 13. In other words the rectangular apertures are bisected by a diameter of the circle defined by the end closing plates 13 and are symmetrically located equidistant on opposite sides of a diameter which is perpendicular to the bisecting diameters. The broad dimensions of the rectangular apertures extend outwardly from the center, of the end closing plates parallel to the bisecting diameter.

The

waves in the window-supporting section 11 to excite the,

desired TE mode consists of two rectangular waveguides 20 and 21 which form astructure desp ribed in the art as a folded, E-plane, T. The structure'is'hesigned to divide electromagnetic energy carried by the single input rectangular waveguide 20 into two equal components which energy may be recombined in the circular partly resonant window-supporting section 11 in such a way as to excite the desired circular TE mode.

An inspection of the mode launcher or transformer 12 illustrated will show that the name used in the art is descriptive. That is to say that the structure of the mode launcher 12 resembles a T. The input rectangular waveguide 20 forms the upright of the T and the power dividing rectangular waveguide 21 forms the upper cross-bar of the T. The upright waveguide 28 extends into one broad wall of the power dividing cross-bar waveguide 21 with its broad wallsextending across the broad wall of the cross-bar. Thus, as illustrated, input power -introduced in the lower part of the upright waveguide 20 enters the power dividing cross-bar waveguide 20 and is divided evenly so that part of the power goes in one direction and part of the power goes in the opposite direction. The cross-bar waveguidev 21 is bent around a narrow wall (an H plane bend) essentially over the upright waveguide to define a U-shape. The ends of the U-shapedcrossbar waveguide 21 extend back and fit in the apertures or slots 14 of one of the end plates 13 of the window supporting structure 11. Thus, the power which enters the upright Waveguide 20 is divided and introduced in the cylindrical window supporting section 11 in the proper phase to excite the circular TE mode. The particular mode :exciter-illustrated has the property of exciting only modes in which the first index digitis even and does not excite odd modes such as the TE mode which might otherwise be troublesome. I

Because it is rarely practical under the present art conditions to consume the output power in the form of the TE circular waveguide mode it is ordinarily necessary to reconvert to one or more rectangular waveguides. In the embodiment of the window section illustratedit is contemplated that the reconversion will take place by means of a mode converter output section which is identical to the input mode converter previously described. Therefore in order to simplify the drawings and description the output mode converter is not illustrated in detail. Electrical reflections inevitably result from the discontinuity at the dielectric barriers. Likewise reflections will occur at one or more points within the associated launching structure. However, by carefully choosing the dimensions of the structure including the radial width of the metallic flanges on the opposite sides of the dielectric barrier, excellent operation is obtained over frequency bands of the order of 20%. That is over a band of frequencies which is 20% of the center frequency for which the device is designed. For example, if the center frequency of the range of interest is 2500 megacycles per second the bandwidth would be on 'the order of 500 megacycles per second extending approximately from 2250 megacycles per second to 2750 megacycles per second.

' An inspection ofthe Window section 10 shows that the dielectric barrier and its supporting structure has complete circular symmetry with respect to both electrical and mechanical properties. This is important because many window failures are traceable to non-uniform electric fields and non-uniform heating resulting in nndesirable stress concentration. Further, the conversion of the energy incident on the dielectric barrier to the "FE circular mode eliminates longitudinal current flow in the cylindrical waveguide. Thus, with respect to the desired circular electric fields a slot into which the dielectric window is fitted (the slot formed by the metallic flanges 16 and17 on opposite sides of the window acts as a strongly attenuating waveguide below cut-oft", so that only very small currents flow around the perimeter of the dielectric barrier. This fact is important because all known techniques for making ceramic to metal seals result in a bond which is a relatively poor conductor and therefore subject to excessive heating when subjected to large values of current.

Since the TE circular waveguide mode is utilized in the cylindrical windowsupporting waveguide section 11 the electric field is everywhere tangential to the surface of the dielectric and is substantially weaker than the field associated with the same amount of power in the normal rectangular waveguide. For example, the relation of power to maximum field in the normal rectangular wave guide is given by the equatiom v is the operating wavelength,

'y is the cutoff wavelength, and where the small dimension of the rectangular waveguide is half the larger one. The corresponding relations for the TE mode in circular waveguide is:

F or specified values of '7, 7 and E, the power handling capacity of the circular guide is 9 times greater than that of the rectangular guide. This-difference is most important a in the gaseous region just outside the dielectric window.

An additional factor of importance is that no component of the electric field is normal to the metal surface in the region of the dielectric '15, therefore, roughness, surface emission, multipactor discharge, and other such effects do not reduce power handling capacity. The fact that the electric field is everywhere tangential to the dielectric barrier leads to the desirable situation that there is no discontinuity of field value and no flux concentration in this region.

FIGURE 3 illustrates the same type of window supporting section 11 as that illustrated in FIGURES 1 and 2 and described above. However, the window-supporting section 11 is utilized with a slightly different kind of mode converter which also excites the TE circular waveguidemode in the window section 11. This mode launcher may be considered an E-pl-ane power divider terminated by a shorting plane. In View of this fact that the window support section 11 is almost identical to that of FIGURES l and 2 corresponding parts of the two devices are given corresponding reference numerals and this part of the Windowsection is not again described in detail. The only difierence in the window support sections of the two embodiments is found in the end plates 13 which essentially close the opposite ends of the sections 11. Each of the end plates 13 of the embod-iment of FIGURE 3 is provided with four power transfer apertures -24 (2 pairs of apertures) instead of the two apertures 14 (one pair) utilized with the end plates 13 in FIGURES l and 2. These apertures 24 are provided to accommodate the different type of mode launcher of this embodiment. The apertures 24- are again rectangular slots positioned radially in the end plates 13. However, each. pair of radial slots is bisected by a diameter which is displaced 90 from the other pair. In other words the mode launcherZS illustrated in FIGURE 3 utilizes four symmetrically disposed feeds instead of the two 'feeds utilized in the symmetrical mode launcher of FIGURES 1 and 2.

The mode launcher 25 is composed of a rectangular input waveguide 26 which divides into two similar parallel waveguides 27 and 28 so that the electromagnetic energy in the input waveguide 26 is equally divided. As illustrated, division of the input rectangular waveguide '26 is In this manner the two parallelpower dividing rectangu lar waveguides 27 and 28 :are formed. These waveguides have approximately the same internal dimensions as the input rectangular waveguide 26. The ends of the two power dividing rectangular waveguides 27 and 28 are closed by a rectangular conductive end closing plate 32 which may bebrazed in place to form a vacuum tight closure if necessary. Also the power dividing waveguides may :be terminated by any of the common means known in the art if reflections of incident power become a problem.

A circular aperture 33 is provided in the upper wall of the mode launching structure 25 to receive one end closing plate 13 of the window supporting section 11. The circular aperture 33 is bisected by the central broad wall 31 of the mode launching structure 25. In other words, half of the aperture 33 extends over each of the power divider rectangular waveguides 27 and 2.8. In order to provide accomplished by tapering the broad walls '29 and 30 outwardly (spread them apart) and in eifect inserting another broad wall septum 31 between the existing broad walls.

for the proper mode excitation in the window-supporting section 11 and the-best power transfer from the mode launcher 25, one end plate 13 (the lower one) is positioned in the aperture 33 with all of the four power transfer apertures 24 located equidistant from the central broad wall 31. Thus, one slot or aperture 24 of each pair of apertures in the end plate 13 is open to one of the rec ta-ngular waveguides 27 and 28. In this manner power in the input rectangular waveguide 26 divides equally in the two power dividing or launching waveguides 2.7 and 28 and excites the circular TE mode in the cylindrical window-supporting section 11. I The end plate 13 is preferably sealed in the circular aperture 33 to provide a vacuum tight closure.

The window section of FIGURE 3 has the same advantages as the window section of FIGURES 1 and 2. For a more complete discussion of the particular type of mode launcher or" FIGURE 3 reference may be had to The Transactions of the IRE, Professional Group on Microwave Theory and Techniques, vol. 'MTT 2#2, July 1954, in the article entitled H Mode Circular Waveguide Components, by T. A. Lanciani, pages 45 51.

Other embodiments of the window or barrier supporting section which are attractive for certain applications are illustrated in FIGURES 4 and 5 and FIGURE 6. It is intended that these structures be used with TE circular mode launchers in a'fiashion similar to that described in connection with previous embodiments. Therefore, the figures are broken away and do not show mode launchers or end plates.

. The cylindrical window-supporting section 4% illustrated in FIGURES 4 and 5 also utilizes a flat dielectric disc 41 as the window or barrier. However, the slot 42 in which the window 41 fits in this embodiment extends outwardly from the circular waveguide body of the window section. As illustrated this window-supporting section. 40 is formed by utilizing one circular waveguide 43 (left in figure) having an outwardly extending flange 44 which in turn has a forwardly extending lip 45. Thus, the lip 45 is cylindrical like the waveguide 43 and is concentric therewith but of a larger diameter. The structure thus formed provides a seat which hold the outer periphery of the dielectric disc or window 41. The mating circular waveguide 46 which makes up the remainder of the cylindrical body of the barrier supporting section 49 has an outwardly extending flange 47 for-med around its outer periphery. This flange 47 butts against the outer edge of the lip 45 and the assembly is held together by some means such as as a braze. Such a braze may provide a vacuum tight seal between the two circular waveguides 43 and 46 and also seal the periphery of the dielectric window 41 in its slot to form a vacuum tight barrier. In connection with forming such joints it should be noted that the outer periphery of the dielectric discs or barriers are usually metallized or.

in which the window 41 is seated performs the same function as described in connection with the previous embodiments. V

A slightly different, embodiment of barrier 50 and barrier supporting structure 51 is illustrated in FIGURE 6.

p In this embodiment the inner diameter of the two circular waveguides 52 and 53 on opposite sides of the barrier 50 difier. However, this is not essential to the use of the type of barrier 50 disclosed in connection with this em face of the window 50 is directed toward the pressurized side. As illustrated, the diameter of the circular waveguide 52' on the concave side of. the barrier 50 is smaller than the circular waveguide 53 on the convex side of the .barrier SO. The waveguides 52 and 53 are provided with abutting flanges 54- and 55, respectively, which are intended to butt against each other and provide a means of brazing the two waveguides together. Since the waveguides 52 and 53 have different diameters the flange 54 on the smaller waveguide 52 extends outwardly and its butting flange 55 extends inwardly on the opposite waveguide 53. r V I The flanges 54 and 55 are shaped so that the outer periphery of the dielectric barrier is held captive in a slot between the flanges 54 and 55 when the two waveguides 52 and 53 are held together. As in the previous embodiments the outer prepihery of the dielectric barrier 50 may be rnetallized for brazing and the assembly may be brazed together in such a manner as to form a vacuum tight seal between the waveguides 52 and 53 and internally between the circular waveguide and the dielectric barrier 59.

. FIGURE 6 has the added virtue that the pressurized side has a larger area, therefore lower electric fields than does the vacuum side. Since gas tends to break down at lower fields than may be readily supported in vacuum this leads to a well balanced design which is economical of materials and space. j

While particular embodiments of the invention have been illustrated and described it will, of course, be understood that-the invention is not limited to these embodiments since many modifications rnay be made in the circuit arrangements and the structures employed. It is contemplated that the appended claims will cover any such modifications as fall within the'true spirit and scope of the invention. a

What I claim is new and desire to secure by Letters Patent of the United States is:

11. A waveguide window section including in combination a Window supporting section and a mode launching means connected to said window supporting section; said window suppor-tingsection including a cylindrical waveguide of circular cross section and a dielectric barrier supported within said window supporting section and sealed therein to provide a vacuurn'tight barrier, and circular conductive end closing members for closing the opposite ends of said cylindrical waveguide, each of said end closing members being provided with a pair of rectangular apertures symmetrically disposed on opposite sides of the center thereof with their major axes on a common diameter, said mode launchingmea-n comprising waveguides of rectangular cross section defining a folded E plane T with the ends of the rectangular waveguide which forms the cross bar of the T inserted in the rectangular apertures of one conductive end plate thereby to launch the circular T E mode in said window supporting section.

2. A waveguide window section comprising: a cylindrical waveguide of circular. cross section; a dielectric barrier supported within said waveguide and sealed thereto to provide a vacuum tight barrier; a circular conductive end member for closing one end of said waveguide, said end member being provided with a pair of apertures therethrough, said apertures being disposed on opposite sides of the-center of said end member; a first rectangular waveguide section bent around a narrow wall thereof to define a -U-shaped structure wherein the broad walls of said first rectangular waveguide section lie in respective parallel planes, the ends of said first waveguide section being joined to said end member to couple electromagnetic energy to respective ones of said apertures, and wherein one of said broad walls is provided with an opening therethrough; and a second rectangular waveguide section, said second section being joined to said one broad wall of said first section to couple electromagnetic energy to said openmg; whereby electromagnetic energy in said second rectangular waveguide section is transmitted through said first rectangular waveguide section to launch the circular TE mode insaid cylindrical waveguide.

3. Thewaveguide window section of claim 2, wherein i said apertures are rectangular in shape.

l 7 References Cited in the file of this patent UNITED STATES PATENTS France Nov. 24, 1 958 

1. A WAVEGUIDE WINDOW SECTION INCLUDING IN COMBINATION A WINDOW SUPPORTING SECTION AND A MODE LAUNCHING MEANS CONNECTED TO SAID WINDOW SUPPORTING SECTION; SAID WINDOW SUPPORTING SECTION INCLUDING A CYLINDRICAL WAVEGUIDE OF CIRCULAR CROSS SECTION AND A DIELECTRIC BARRIER SUPPORTED WITHIN SAID WINDOW SUPPORTING SECTION AND SEALED THEREIN TO PROVIDE A VACUUM TIGHT BARRIER, AND CIRCULAR CONDUCTIVE END CLOSING MEMBERS FOR CLOSING THE OPPOSITE ENDS OF SAID CYLINDRICAL WAVEGUIDE, EACH OF SAID END CLOSING MEMBERS BEING PROVIDED WITH A PAIR OF RECTANGULAR APERTURES SYMMETRICALLY DISPOSED ON OPPOSITE SIDES OF THE CENTER THEREOF WITH THEIR MAJOR AXES ON A COMMON DIAMETER, SAID MODE LAUNCHING MEANS COMPRISING WAVEGUIDES OF RECTANGULAR CROSS SECTION DEFINING A FOLDED E PLANE T WITH THE ENDS OF THE RECTANGULAR WAVEGUIDE WHICH FORMS THE CROSS BAR OF THE T INSERTED IN THE RECTANGULAR APERTURES OF ONE CONDUCTIVE END PLATE THEREBY TO LAUNCH THE CIRCULAR TE01 MODE IN SAID WINDOW SUPPORTING SECTION. 